Biorhythm is terrible in Limited too but they print plenty of bad Limited cards on bonus sheets, so its definitely possible
For these decks, the opportunity of playing Jegantha, the Wellspring is just too high, not because of the number of cards they can't play, but because of the power-level of the small number of cards that don't meet Jegantha, the Wellspring companion restriction
On top of that, additional card advantage through a Vehicle dealing combat damage is just the cherry on top
You gain control of an opponents permanent when it enters and attack with it to remove a stun counter
Well want to pick one that buffs our creatures to get more value from Shadowheart
There are definitely combos possible here as long as the cauldron stays in play for its static effect and you can build your deck to take advantage of this card and have opponents just be a bonus